Frequency shift keyer



Patented July 12, 1949 FREQUENCY SHIFT KEYER De Witt Rugg Goddard, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 21, 1946, Serial No. 692,095

9 Claims.

This application concerns signaling systems of the telegraph, facsimile, etc. type wherein carrier current has its frequency shifted between two frequency values representing different signaling conditions such as, for example, mark and space in telegraphy or black and white in facsimile.

The general object of the present invention is improved frequency shift signaling as outlined briefly hereinbefore.

In signaling systems of this type it is important not only that the carrier frequency can be shifted as desired between two frequency values in accordance with signals but that the center, average, or mean frequency of the carrier be substantially fixed; otherwise, interference between channels may take place. This suggests the use of stabilized oscillators, such as crystal controlled oscillators, and the same have been used heretofore in systems of this type. More specifically heretofore several methods have been used to achieve frequency shift keying of crystal controlled transmitters. One method is to utilize in a crystal controlled oscillator, a crystal ground to, say, 200 kilocycles below the frequency of the crystal normally used in the said transmitter. Then by means of a detector or mixer a carrier of 200 kilocycles from a local high stability reactance tube mudulated 200 kilocycle oscillator,

shifted in frequency in accordance with the signal, is added to the oscillations from the said crystal oscillator to bring the resultant frequency (sum) back up to the required value.

An object of my invention is to provide a frequency shift system of the general type described herein'before wherein the crystal normally used as a frequency controlling medium in the transmitter may be used Without change when the said transmitter is adapted for frequency shift signaling. The advantage of such an arrangement will be obvious to those skilled in the art at once. One advantage resides in elimination of the need of providing a new crystal in the transmitter.

A further object of my invention is to provide in a system of the general type described hereinbefore a greater frequency spacing between the crystal oscillator frequency and undersired modulation components. Attainment of this object greatly simplifies the filtering circuits necessary in this system. As a matter of fact, in said embodiments of my invention simple tuned circuits are all that are required to select side band components to be used and discard components not to be used. An additional advantage which results from attainment of the two objects listed 2 above is that a frequency shift unit arranged in accordance with my invention may be applied to any crystal controlled transmitter in a few minutes without provision of special crystals for use therein.

Briefly, the above objects are attained in accordance with my invention by providing an oscillator which is highly stable and yet which may be modulated or shifted in frequency in accordance with signals. The modulated oscillations, directly or after mixing with other fixed frequency oscillations of a frequency equal to the center frequency of the modulated oscillations, are mixed with output from the transmitter crystal controlled oscillator and the sum frequency selected. The said other fixed frequency oscillations may be of relatively low frequency and may be derived from a crystal oscillator, and these or similar oscillations are then supplied to a mixer, also excited by the selected upper side band. The difference frequency is selected from this mixing action and this difference frequency will be the same as the original transmitter crystal frequency, but this selected side band will be shifted in frequency in accordance with the modulation on the first modulated oscillator.

In a modification, to extend the frequency spacing between the side bands, the output of the modulated oscillator and the crystal oscillator which supplies the oscillations of fixed frequency equal to the center frequency are intermodulated or mixed, and the sum frequency selected for mixing with the output from the transmitter crystal oscillator, and the sum frequency is again selected from this last mixing. The output of the low frequency crystal oscillator is then doubled and supplied along with the last-named selected sum frequency to another mixing stage wherefrom the difference frequency is taken. The side bands in this embodiment are spaced from each other by an amount double the amount of spacing between the side bands in the first embodiment. A further spacing between the side bands is accomplished in a third embodiment by supplying the first mentioned selected sum frequency to a mixer also excited by oscillations of double the frequency of said low frequency crystal oscillator. Then the upper side band is selected and fed to a mixer also excited by the output from the transmitter crystal oscillator. The sum frequency is again selected from this last mixing and supplied to a mixer also excited by oscillations of a frequency four times the frequency of said low frequency crystal oscillator and the difference frequency selected.

In Fig. 1 of the drawings, A represents a reactance tube modulator of any known approved type controlled by keyed potential from a line L to shift the frequency of a 200 kilocycle oscillator B back and forth with respect to its unm'odulated frequency. The 200 kilocycle oscillations shifted in frequency in accordance with signals are fed from B to a mixing stage or detector D. Detector stage D is also fed with a 200 kilocycle signal from a crystal controlled oscillator in C operating at 200 kilocycles per second. The output of this detector D contains various modulation components such as audio frequencies eorrespendingv to. keying speeds, audio frequencies between the 200 kilocycle oscillators, 200 kilocycles and 400 kilocycles, with the. frequency shifted modulation imposed on it; Filter E selects this latter upper side band component, discarding the others. This 490 kilocycle component is fed to a second detector or. mixer stage G. Detector stage G also receives the output of a crystal controlledoscillator F which, is part of the transmitter equipment. Crystal oscillator F has for its crystal the one normally used by the transmitter as a frequency controlling medium.- For the sake. of the. present explanation it is as.- sumed that. the crystal i F has. a frequency of 2.2 megacycles. per second. The output of detector G contains various components such as. L00. kilocycles with frequency shift modulation, the difference between. 2.2. megacycles and .4 'megacycle. or. 1.8 megacycles... 2.2 megacycles and the sum of the. two. exciting frequencies or 2.6 megacycles. All these. frequencies. are fed to the filter I which is arrangedv to pass only the. 2.6 megacycle component which. contains the frequency shift modulatiorr and discards or suppresses. all other components The output of filter Iv feeds. the. detector J. Detector J is. also supplied by oscillations of 40.0 kilocycles per. sec.- ndv from a, doubler in unit H which is excited by oscillations. of. 20.0 kilocycles per second from the crystal oscillator C. The. output of the .detector J contains. the. sum and. difference frequency components. of the. applied frequencies and also the. applied frequencies, themselves. In this case filter Kselects the. difference. component between 2.6 megacyclesper second and 4.00. kilocycles per second. which is. 2.2 .megacycles or the same frequency as that generated by the original transmitter crystal in. unitF. In this case... how.- eyer, the 2.2 megacycles. per second oscillations are shifted in frequency in accordance with the signals at L and on the output of the oscillator at B...

Consideration of the embodiment described above will show that in thesystem I. provide a 200 kilocycl-e oscillator of high stability wherein the modulation or frequency shift in. accordance with signals is carried out. and. yet I havea 40.0 kilocycle per second carrier shiftedinfrequency in accordance with the signals for modulation of, the transmitter crystal frequency. This facilitates design of the. filters l and K. making the. same quite simple. In fact, they may in many cases be simple tuned, circuita.

Where still. greater frequency separation is desired, the embodiment of. Fig. 2 may be. used. In this embodiment, the. transmitter oscillations of 2.2 megacycles are modulated by a 800 kilocycle per second carrier in. turn shifted infrequency in accordance with signals. To. obtain this separation the output of the 'filter'E'is fed to .amixer. M. The output of the doubler H. is also connected to this mixer M and the upper side band is selected 7 from the beat.

collected at the output from the mixer or detector M output and supplied to the detector or mixer G. Note that in the output of the mixer M the sum frequency is spaced from the difference frequency by 800 kilocycles which in percentage is so much as to eliminate the need of filters other than resonant circuits tuned to the frequency of the. said upper side band. The doubler H output is also supplied to an additional doubler H and thence to the detector J wherein it is mixed with the filtered upper side band selected from the output of the detector G and filter I. In the example given, the center frequency of this upper side band is 3 megacy-cles per second. The lower side band is of detector J by filter K and will have a. frequency of 2.2 megacycles per second plus and minus the frequency shifts in accordance with signals. Note in this embodiment there is still greater frequency separation between the desired frequencies put. to use and the rejected components.

In some cases. the detector D, doubler H and filter E of the embodiment illustrated in Fig. 1 may be omitted. Then my improved frequency shift modulator is as illustrated in Fig. 3 of, the drawings. The output of the 200 kilocycle modulated oscillator B is fed directly to. the. detector or mixer G andthe output of thev 2.00 kilocycle crystal oscillator C is fed directly to. the. detector or mixer J- This em'boclmient of .Fig. 3 provides a less. complicated .set. up but the desired and undesired components produced by detectors G- and J are not so widely separated in they frequency spectrum For example. the. main side bands are separated by 200 kilocycles only, More care, must be given to. clesignan-d proportioning of the. filter networks .I and K in order of. select for use; the desired. .side band and discard the remaining modulation components.

The embodiment.- of Fig. 3. may be arranged to provide. 400. kilocycles separation between, the main side band. components at the output of; the detector J by using at B a. stabilized modulated source having a center or average. frequency of 40.0 kilocycles per second and a crystal controlled oscillator at C. operating at400 kilocycles per sec.- end. When operated-in this manner, however, less tabilibity is obtained inthe arrangement of Fig. 3, due to the well-known fact that the absolute. stability of an oscillator improves as the frequency is lowered.

The oscillators, detectors or mixers, filters and frequency multipliers in all of the modifications may be. conventional and may follow prior art oscillators, detectors,, filters and frequency multipliers. For this reason, and for the sake of simplicity, the same .have been shown merely by rectangles.

What. is. claimed is:

1. A frequency shift keyer including in combination, means for generating oscillatory en.- ergy of relatively high frequency, and of substantially fixed frequency, means for generating relatively low frequency oscillatory energy the frequency of which is shiftedin accordance with signals, means for generating oscillatory energy of substantially fixed frequency substantially equal to the mean or average frequency of said last. mentioned oscillatory energy, a first mixer stage wherein oscillations representing said frequency shifted oscillatory energy are mixed with oscillations representing said oscillatory energy of high. frequency. means for selecting the; upper side, band resulting, from said. first mixer stage, a second mixer stage excited by oscillations representing said generated oscillatory energy of fixed frequenc equal to said center or average frequency and by said selected upper side band, and means to select the difference frequency from said last mentioned mixer stage.

2. A frequency shift keyer including in combination, means for generating oscillatory energy of relatively high frequency and of substantially fixed frequency, means for generating relatively low frequency oscillatory energy the frequency of which is shifted in accordance with signals, means for generating oscillatory energy of substantially fixed frequency substantially equal to the mean or average frequency of said last men tioned oscillatory energy, a first mixer stage wherein oscillations representing said frequency shifted oscillatory energy are mixed with oscillations representing said oscillatory energy of a frequency equal to said average frequency, means for selecting the upper side band from said first mixer stage, a second mixer stage excited by said generated oscillations of relatively high frequency and by said selected upper side band, a third mixer stage coupled to said second mixer stage, a frequency doubler coupling said third mentioned generating means to said third mixer stage. and means to select the difference frequency from said third mixer stage.

3. A frequency shift keyer including in combination, means for generating oscillatory energy of relatively high frequency and of substantially fixed frequency, means for generating relatively low frequency oscillatory energy the frequency of which is shifted in accordance with signals, means for generating oscillatory energy of substantially fixed frequency substantially equal to the mean or average frequency of said last mentioned oscillatory energy, a first mixer stage wherein oscillations representing said frequency shifted oscillatory energy are mixed with oscillations representing said oscillatory energy of a frequency equal to said average frequency, means for selecting the upper side band energy from said first mixer stage, a second mixer stage cou-- pled to said last named means and excited by said upper side band energy, a frequency doubler coupling said third generating means to said second mixer stage, a third mixer stage coupled to said second mixer stage, and to said first generator, a fourth mixer stage coupled to said third mixer stage and to said frequency doubler by a second frequency doubler, and means to select the lower side band produced by said fourth mixer stage.

4. A frequency shift keyer including in combination, means for generating oscillatory energy of relatively high frequency and of substantially fixed frequency, means for generating relatively low frequency oscillatory energy the frequency of which is shifted in accordance with signals, means for generating oscillatory energy of substantially fixed frequency substantially equal to the mean or average frequency of said last mentioned oscillatory energy, a first mixer stage wherein oscillations representing said frequency shifted oscillatory energy are mixed with oscillations representing said oscillations of relatively high frequency, a filter coupled to said mixer stage for selecting the upper side band resulting from said mixer stage, a second mixer stage excited by oscillations representing said generated oscillatory energy of fixed frequency equal to said center or average frequency and coupled to said filter to be excited by said selected upper side band, and a second filter coupled to said second mixer stage to select the difference frequency therefrom.

5. In a frequency shift telegraphy system in combination, a crystal controlled oscillator operating at high frequency, a low frequency oscillator modulated in accordance with signals, a second low frequency oscillator of a substantially fixed frequency substantially equal to the average or mean frequency of said modulated oscillator, a mixer coupled to said last named oscillators, a second mixer coupled to said high frequency oscillator, a filter arranged to pass the upper side band coupling said first mixer to said second mixer, a filter tuned to the upper side band coupled to said second mixer, a third mixer coupled by a doubler to said third mentioned oscillator and to said second filter, and a filter tuned to the lower side band coupled to said last named mixer.

6. The method of frequency shift signaling which includes these steps, modulating the frequency of oscillatory energy of relatively low carrier frequency in accordance with signals, providing oscillatory energy of a substantially fixed carrier frequency substantially equal to the average frequency of said modulated energy, modulating relatively high frequency oscillatory energy by oscillations characteristic of said modulated oscillatory energy, selecting the upper side band of the resulting modulation components, intermodulating oscillations characteristic of said provided oscillatory energy and said selected side band, and selecting the lower side band of the modulation components resulting from said last intermodulation step.

'7. The method of frequency shift signaling which includes these steps, modulating the frequency of oscillatory energy of relatively low carrier frequency in accordance with signals, providing oscillatory energy of a substantially fixed carrier frequency substantially equal to the average frequency of said modulated oscillatory energy, intermodulating said modulated energy and said provided oscillatory energy, selecting the upper side band of the resulting modulation components, modulating a relatively high frequency carrier by said selected side band, selecting the upper side band of the resulting modulation com ponents, doubling the frequency of said provided oscillatory energy, intermodulating said last mentioned selected side band and said oscillatory energy of doubled frequency, and selecting the lower side band of the modulation components resulting from said last intermodulation step.

8. The method of frequency shift signaling which includes these steps, modulating oscillatory energy of relatively low carrier frequency in accordance with signals, providing oscillatory energy of a substantially fixed carrier frequency substantially equal to the average frequency of said modulated energy, intermodulating said modulated energy and said provided oscillatory energy, selecting the upper side band of the resulting modulation components, doubling the frequency of said provided energy, intermodulating said energy of double frequency and said selected side band, selecting the upper side band resulting from said last intermodulation step, modulating a relatively high frequency carrier by said last mentioned selected side band, selecting the upper side band of the resulting modulation components, multiplying the frequency of said provided oscillatory energy by four, intermodulating said last mentioned selected side band and said oscillatory energy of multiplied frequency, and selecting the lower side band of the modulation components resulting from said last intermedulation step.

' 9. In apparatus for generating side band signalling energy of substantially fixed and known frequency, the frequency of which is shifted in accordance with signals, in combination, a crystal controlled oscillator operating at a relatively high and substantially fixed frequency substantially equal to the average or center frequency of the side band to leev generated, a source of oscillatory energy of relatively low frequency, apparatus including a modulator coupled to said last named source for shifting the average or center frequency of the energy in accordance with signals, circuits including a. first mixing and converting stage coupled to said last apparatus and to said source of relatively high frequency oscillatory energy for modulating the high frequency energy by the frequency shifted energy, a filter coupled to said first mixer for selecting therefrom an upper side band, a source of oscillatory energy of substantially fixed frequency substantially equal to the average or center frequency of said selected side band, a second mixing and converting stage coupled to said filter to be excited by said upper side band, and a coupling between said second mixer and said last named. source :of energy of relatively fixed frequency for impressing on said second mixer oscillatory energy of a substantially fixed frequency equal to the average frequency of said selected side band, and a filter coupled to said second mixer and tuned to select the lower side band from the output of the second mixer. DE WIT'I RUGG GODDARD.

REFERENCES CITED The following referenlces are of record in the file of this patent:

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